1. Field of the Invention
The present invention concerns a method for evaluation of two corresponding images of an organ acquired at different points in time as well as a medical imaging system for the same purpose.
2. Description of the Prior Art
Longitudinal imaging examinations are necessary for clarification of many medical questions. This means that an organ, organ system or body part to be examined is imaged multiple times at different points in time with the same imaging method. For example, the progression of a specific illness can be determined via a comparison of the images of the organ. Often specific diseases (primarily degenerative illnesses) can only be detected or at least supported via a change that manifests in an organ over the course of time.
One example of such an illness is Alzheimer's disease. This illness can be diagnosed only with difficulty with conventional examination methods, in particular when it is in its early stage. This illness is among the neurodegenerative illnesses and it is known that an atrophy in specific brain regions appears over the course of the Alzheimer's disease due to the loss of nerve cells in, among other places, the grey brain matter (also called the cerebral cortex) of the parietal lobe and temporal lobe.
Although the occurrence of these changes is a known fact, imaging methods (particularly MRT (magnetic resonance tomography) are only very conditionally suitable for diagnoses of Alzheimer's disease, even when volumetric methods are used for measurement of the size of specific cortical regions. This is partially due to the fact that dimensions of the atrophy to be measured and detected lie quantitatively below the inter-individual fluctuation range of the size of specific brain regions.
One possibility to counteract this is to examine the brain of a patient at various points in time and to establish the progression of the atrophy through a comparison of the images. This method, however, has the disadvantage that the differences to be detected cab be very slight, such that a diagnostician who compares the images can easily overlook the differences.
Moreover, when images have been produced by means of MRT methods (typical in neurology) the images exhibit the peculiarity that the images produced at different points in time can exhibit different distortions (particularly deformations). This is due to the fact that MRT methods, in which different magnetic fields that are precisely tuned to one another are used for imaging in a known manner, react sensitively to interfering influences. Therefore, even if care is taken to ensure precisely the same acquisition conditions when the respective images are acquired, the different respective images still will not always exhibit the same (and therewith comparable) geometric deformations. Only the geometric deformations were detected and compensated by means of measurements of a phantom produced before the acquisition could the geometric deformations be better compensated. This is not done in practice, however, due to the large effort and the costs that would be associated therewith.
It is therefore normally not possible for a diagnostician to decide whether the slight but diagnostically relevant differences in the various images are to be ascribed to a geometric deformation or to an actual change of the anatomical conditions in an organ. Due to these facts, MRT examinations previously have not belonged to the recognized and established methods of Alzheimer's diagnostics, but rather are primarily used for exclusion of other illnesses.
Various approaches have been proposed in order to counter the aforementioned problems. A method for measurement of volume changes given repeated three-dimensional MRT acquisitions is disclosed in the article by Freeborough, P. A., Fox N. C., “The boundary shift integral: an accurate and robust measure of cerebral volume changes from registered repeat MRI”, IEEE Trans. Med. Imaging 1997; 16: 623-629. The MRT exposures produced at different points in time are rigidly aligned relative to one another and their intensity value differences are integrated. Volume differences of the entire brain can in fact be detected with this method, but this method does not take into account different geometric deformations. Additionally, volume changes cannot be associated with specific brain regions, such that this method often provides a diagnostician with insufficient assistance in the finding or exclusion of a specific diagnosis.
Modifications of this method are known wherein only specific brain regions are examined in order to obtain at least a rough localization of the morphological variations, but here as well geometric deformations are not taken into account.
Although the above problem has been described in the context of Alzheimer's disease and its diagnosis by means or MRT, similar problems exist in the case of other medical questions. The progress of a tumor illness or osteoporosis is one example. The problems of a geometric deformation illustrated above likewise arise predominantly in an MRT examination; however, other examination modalities (such as, for example, computed tomography) can also exhibit similar problems, for example given an incorrect calibration.